By attempting to understand the root causes of past global warming events, can …

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Although it's widely acknowledged that global warming is happening, the
long-term effects still remain uncertain. In an attempt to
understand this, scientists are looking to the past for an answer. The
earth has experienced large, rapid warming trends due to greenhouse
gases in the past, and they may prove to be an analogue to what is
happening in
the world around us today.

Around 55 million years ago, the Earth experienced a rapid warming
period that lasted for almost 200,000 years. This warming, known as the
Paleocene-Eocene Thermal Maximum (PETM) has been proposed as an
analogue of what is occurring today. Temperature records dating from the
early period of the PETM showed that temperatures all over the globe
rose by approximately 5 oC in as little as
10,000 years. This was
accompanied by a change in the global hydrological cycle and a decrease
of the 13C/12C
ratio
found in carbon-containing organisms both from the
land and sea. This change in isotope ratio suggests that the surge in
global temperatures was linked to a change in the carbon cycle. During
this period there were mass
extinctions of species, as well as others spreading about the globe.
This time period was
also accompanied by the appearance of modern orders of mammals,even up to primates. Although scientists know a great deal about this
period of rapid global warming—that it was caused by a massive amount
of CO2 in the atmosphere, altered the
carbon cycle, and greatly
altered ecosystems—one thing remains unclear: what set this off? What was
the original source of all this carbon?

One hypothesis suggests that the PETM was triggered by a release of about
2000 PgC (petagrams C) from methane hydrates under the sea; however,
calculations suggest that this is nowhere near enough carbon to
trigger such a sharp and sustained rise in temperature. Other
additions and suggestions to the hypothesis list include oxidation of terrestrial and
marine organic carbons, as well as volcanic eruptions and decaying
organic matter, although there is no one completely accurate and
satisfactory explanation. New work reported in the latest issue of Science tried to answer some questions about the root cause of the PETM. By using conventional values for
climate sensitivity—1.5 - 4.5 oC per
doubling of atmospheric
CO2
concentration—along with estimates of pre-PETM atmospheric CO2
concentration levels, the researchers postulated that an increase of 750
to 26,000 ppm of CO2 would be needed to give
rise to the 5 oC
temperature increase seen during this time. This
concentration increase corresponds to an extra 1500 to 55,000 PgC in
the atmosphere alone, but these high carbon levels and temperatures
were sustained over 100,000 years. In order to sustain such a large
amount of carbon in the atmosphere for such long time scales, an
equilibrium must be reached between atmospheric and oceanic carbon
levels. Taking that
into account suggests that a total carbon release of between 5400 and
112,000 PgC would have had to occurred, a value that dwarfs the
estimated
5000 PgC in sequestered fossil fuels available today.

Clearly this value is enormous, and suggests that methane hydrates
alone were not solely responsible for this
well documented 5 oC temperature rise.
The other possibility
here is that our estimates for climate sensitivity to CO2
levels are
off, according to the authors of the article

" ... the PETM either resulted from an enormous input of CO2
that currently
defies a mechanistic explanation, or climate sensitivity to CO2
was
extremely high."

The authors discuss the next challenges in solving this mystery, and
suggest that they will get a more accurate answer if they constrain the
magnitude and rate of the carbon being introduced, allowing them to
more accurately model this CO2-induced global
warming event. With a
more accurate model, the authors hope to find out if this past global
warming can be used to help us further our understanding of what is
occurring in the world around us today.

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Matt Ford
Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems. Emailzeotherm@gmail.com//Twitter@zeotherm